Telecommunications repeaters or regenerators and in particular optical repeaters, require associated supervisory circuitry to monitor operation and to ensure that the device is functioning correctly. Output signals from the supervisory circuit are impressed on the output signal from the repeater or regenerator as a phase modulation of the digital data stream. This modulation may be achieved by phase modulation of sinusoidal signals generated by a clock within the repeater. Conventional phase modulation circuits suffer from the disadvantage that their output signal varies significantly in amplitude. Under the stringent conditions of repeater operation this amplitude variation is considerable.
The object of the present invention is to minimise or to overcome this disadvantage.